flux-shell: support stdin flow control to subprocesses

[chu11]

libsubprocess now supports flow control on writes to ensure no buffer overflow

This is supported in flux-exec (#6370) but not in the flux-shell when data is read from the KVS. #6292 is probably a necessity to get this to work.

Side note, stdin via file in flux-shell simply dups the file descriptor to the subprocesse's stdin. So buffer overflow should not be possible when using file input for stdin. So there is a decent workaround to buffer overflow in flux-shell that can be handled right now.